The long-term objective of this research is to describe the mechanisms of development and cell differentiation at the molecular level. The Drosophila eye is chosen as the experimental system. This proposal outlines experiments aimed at deciphering the role of the asteroid gene in eye development. Asteroid cDNAs will be isolated and the DNA sequence of the coding regions of the gene determined. This sequence will provide the protein sequence of the asteroid gene product and identify its cellular function. The cellular defect in the eye caused by asteroid mutations will be examined by light microscopy of adult eye tissue sections to identify missing or aberrant cell types. The timing of the developmental mishap caused by the lack of asteroid function will be investigated by light microscopy of whole-mounted imaginal discs that have been stained with antibodies to neural epitopes or to specific gene products expressed in the cells of the eye. The specific cell types that are missing or develop aberrantly will be identified by electron microscopy of sectioned immunohistochemically stained discs. A genetic mosaic analysis will be done to determine if asteroid is cell autonomous and to determine in which cells of the eye the asteroid gene product is needed for normal development. Finally, an asteroid-LacZ fusion construct will be made in an expression vector and used to produce monoclonal antibodies to the asteroid protein. These antibodies will then be used to localize the asteroid protein in the cells of the eye. These experiments will determine the role of the asteroid gene in eye development and suggest mechanisms by which this role is carried out. This research will be used as a starting point to investigate how asteroid interacts with other gene products in an informational pathway to determine cell fate and how it functions in the development of the entire organism.